1. Field of the Invention
The present invention relates to a rear projection lens device for facilitating a zoom adjustment and a magnification adjustment thereof.
2. Description of the Related Art
In recent years, a large-screen television, which utilizes a rear projector, has become widely used. The rear projector has a translucent screen incorporated in a housing. A plane mirror, which is installed behind the screen, and a projector unit for projecting image light to the plane mirror are accommodated in the housing. The projector unit comprises a projection lens device, an image display panel and an illumination lamp installed in this order from the screen side. The illumination lamp illuminates the image display panel, so that an image displayed on the image display panel is enlarged by the projection lens device to be projected onto the back of the screen. The image formed on the back of the screen is seen from the front side of the screen.
Plural types of the rear projectors are manufactured to the same aspect ratio but different screen sizes. Such rear projectors are different in projection distance between a display surface of the image display panel and the screen, and display field angle (projection angles with respect to the diagonal length of the screen) besides the size of the housing. Therefore, conventionally, the plural types of the projection lens device, which are different in the display field angle, are supplied to deal with the various screen sizes. However, the costs increase when the plural types of projection lens device are manufactured in different specifications. For that reason, it is desired to use a common projection lens device regardless of the screen size.
To achieve commonality of the projection lens device, a zoom adjusting mechanism, which changes the projection angle according to the screen size, becomes necessary. The rear projector has shortened projection distance and significantly enlarged projection magnification to reduce the depth of the projection lens device. For instance, a frame of one square inch is enlarged to the size of the screen (50-60 square inches, for instance). Therefore, recently, a wide-angle aspheric lens is used for the projection lens device.
Usually, when focusing on a subject in a short distance, a lens system is moved near the subject. In this case, however, a sharp image cannot be attained due to aberration variations, especially spherical aberration, which is caused by curvature of field of the lens system. For that reason, the projection lens device requires a floating mechanism for correcting such aberrations. It is also desirable to provide the floating mechanism to the rear projection lens device with a zoom adjustment mechanism, since the aberration correction becomes necessary when the lens system is moved close to the screen for the zoom adjustment.
Japanese Utility Model publication No. 63-6722 and Japanese Patent No. 3450670 respectively disclose a floating lens device with the floating mechanism. The floating lens device has a front lens group and a rear lens group. The floating lens device has a structure that the front lens group is moved forward in accordance with a range ring (a rotation ring), which is operated when focusing on a near-distance subject, and the aberration is corrected by moving the rear lens group relative to the front lens group in response to the movement of the front lens group.
However, the floating lens devices disclosed in the Japanese Utility Model Publication No. 63-6722 and Japanese Patent No. 3450670 are for cameras, so the structure of the lens device including the floating mechanism is complicated under constraints: the floating mechanism is activated at focusing, the range ring is rotated in a fixed position, and a lens barrel cannot be long. On the contrary, parts, such as the range ring, are not necessary for the projection lens device, since there is no need of readjusting zoom and focal positions once they are determined. The rear projection lens device would become expensive if the conventional floating mechanism is incorporated; therefore, the projection lens device with the floating lens mechanism is not yet in the actual use.